The present invention relates to a light controlling device having an excellent controllability of light transmittance and a process for controlling light transmission using the device. More particularly, the invention relates to a light controlling device using a light-transmissive non-liquid medium for dispersing particles therein, and a process for controlling light transmission using the device.
In recent years, multistory buildings increase more and more, and moreover, the windows have become a large size. At least 25% of the energy consumption in office buildings is energy that comes in or out as a light or heat through windows. Accordingly, from the viewpoint of energy saving or effective utilization of energy, control of light transmission of window glasses, particularly development of large-area light controlling device applicable to windows, is demanded. Also, at present where the number of automobiles increases owing to rise of motorization, and in addition, where most of automobiles are equipped with air conditioner with trend toward high-class motorcars, it is demanded, from the viewpoint of effective utilization of energy, to develope a light controlling device suitable for windows of electric motorcars which particularly requires severe restriction of energy consumption.
In order to meet the demands, there has been developed light controlling devices, e.g. devices for electrically controlling the absorbance or reflectance to sunlight intended to reduce air conditioning load. Studies of some electromagnetic optical elements have also been made for the purpose of application to light controlling devices.
One of light controlling devices which have hitherto been studied is an electrochronic element (EC element), as known from C. M. Lampert, Solar Energy Mater., Vol. 6, No. 1, 1981. The light controlling device using the EC element utilizes a change in spectrum resulting from an electrochemical oxidation-reduction reaction of a tungsten oxide film or the like, namely a large absorption or reflection of tungsten bronze to light in the region of visible rays to near infrared rays.
As another light controlling device, there is known a device using a suspension of anisotropic dipole particles in a liquid dielectric medium (the device being thereinafter referred to as "DPS device"). The DPS device basically composed of a suspension of fine particles having a shape anisotropy, e.g. needle-like or plate-like particles, in a liquid dielectric, which is sandwiched between a pair of glass substrates having a transparent electrically conductive film. The DPS device utilizes a change in light transmittance resulting from varied orientation of dispersed particles in response to application of an electric field to the suspension. The fine particles orient in the state that their major axes are parallel to the direction of the electric field when the electric field is applied to the suspension, thus transmitting light well (open state). When the action of electric field is released, the orientation of particles is released by Brownian movement and the particles orient randomly, thus decreasing light transmission (closed state). This state change in electromagnetic optical devices is called a switching action.
The electromagnetic optical devices using the DPS element began to be developed from around the end of 1960 for the original purpose of applying to display element, and proposals relating thereto are made, for example, in U.S. Pat. No. 3,257,903 and Japanese Patent Publication Kokai No. 61-69038. This electromagnetic device can be used as light controlling device since light transmittance changes under an electric field.
The light controlling device using the EC element has the defect that because the EC element is electric current actuation type, it causes IR drop and an increase in the area of the device results in a marked decrease in speed of response. It has also a problem owing to working principle that the EC element does not substantially operate at low temperatures. These problems are a serious obstacle to application to large-area light controlling device.
The light controlling device using the DPS element shows a large charge of light transmittance in the initial stage of use and is relatively easy to produce devices having a large surface area because of electric field actuation type. However, in general, this device has the problem that because fine particles are dispersed in a liquid medium, the particles coagulate and precipitate, thereby causing color irregularity in the vertical direction, decrease in degree of change of light transmittance and worsening of switching action.
Also, as a related art, a multi-directional light control film including needle-like particles having their major axes aligned in parallel within a matrix is proposed in EP-A-0,403,116. The alignment of the needle-like particles is completely locked in the matrix. Accordingly, the proposed multi-directional light control film has an anti-glare effect, but does not have an ability of arbitrarily controlling light transmittance, i.e. so-called light controlling ability.
It is an object of the present invention to provide a light controlling device of a different type from electric current actuation type and electric field actuation type, namely a device which can control light transmission without applying an electric current or an electric or magnetic field.
A further object of the present invention is to provide a light controlling device which has no problem resulting from agglomeration or precipitation of fine particles dispersed in a light transmissive dispersing medium.
Another object of the present invention is to provide a process for controlling light transmission without application of an electric current or an electric or magnetic field.
These and other objects of the present invention will become apparent from the description hereinafter.